Field of the Invention
The present invention relates to a secondary cell of an inorganic solid excellent in safety and environmental resistance, based on an operation principle of forming a new energy level in a band gap utilizing photoexcited structural change of a metal oxide caused by ultraviolet irradiation and capturing an electron.
Description of the Related Art
Amid growing awareness of global environmental problems such as exhaustion of fossil fuels and warming associated with increases in carbon dioxide, attention is being given to solar cells as clean energy sources and secondary cells for storing energy.
In particular, automobiles are powered by fossil fuel, emitting hazardous substances such as carbon dioxide; therefore, reductions in fuel consumption and exhaust gas are an important issue. Accordingly, HEV (Hybrid Electric Vehicle) which is also driven by an electric motor and EV (Electric Vehicle) which is fully electrified are expected to become mainstream.
Further, the latest mobile devices have become remarkably widespread, and the charge power source capacity is expected to increase.
In the current HEV, EV, or mobile devices, a nickel-hydrogen secondary cell is commonly used as a secondary cell. Recently, a lithium-ion cell has been developed as a secondary cell capable of higher output and larger capacity, and is in the initial stage of practical use.
In the lithium-ion cell, a metal double oxide containing lithium is used as a positive electrode and a material such as carbon that can accept and release lithium is used as a negative electrode, which are impregnated with an electrolytic solution containing a lithium salt capable of ionic dissociation and an organic solvent capable of dissolving it. Carbon electrodes of graphite powder improved for higher performance and larger capacity are disclosed (e.g., see Japanese Patent Application Laid-Open No. 2002-124256 (Patent Document 1), Japanese Patent Application Laid-Open No. 2002-141062 (Patent Document 2), etc.). Further, there is also a case where fibrous carbonaceous matter as a conducting agent is contained in a sheet-shaped negative electrode and thermoplastic resin is used as a binding agent, thereby providing the sheet-shaped negative electrode for a high-performance lithium-ion secondary cell at low cost (see Japanese Patent Application Laid-Open No. 2009-146581 (Patent Document 3) etc.)
In the case of using a transition heavy metal oxide as the active material of the positive electrode of the lithium-ion cell, the high specific gravity of the element theoretically makes it difficult to produce a large-capacity cell. For this reason, there is disclosed a high energy density, large-capacity, and high-stability cell utilizing a radical compound as the active material of an energy storage device such as the cell by stabilizing the radical compound produced by a radical reaction in the course of at least one of charging and discharging as electrochemical oxidation-reduction reaction (see Japanese Patent Application Laid-Open No. 2002-170568 (Patent Document 4) etc.)
Further, there is disclosed an all-solid lithium secondary cell using a solid electrolyte in place of an electrolytic solution because there is a possibility of liquid leakage due to the electrolytic solution being liquid and it is necessary to enhance the safety of the cell in improper use due to the use of a combustible (see Japanese Patent Application Laid-Open No. 2007-5279 (Patent Document 5) etc.)
The lithium-ion cell is expected to be a secondary cell of high performance and large capacity, in comparison with the conventional nickel-hydrogen secondary cell. However, at present, for example, the travel distance of EV is about 100 km, and it is necessary to further increase the capacity of the secondary cell. Further, the cost of the secondary cell is about half that of EV, and is also required to be lowered.